1. Field of the Invention
The present invention relates to an image pick-up apparatus such as a digital camera that picks up a static image, and particularly relates to improvement of a shutter unit.
2. Description of the Related Art
An image pick-up apparatus such as a digital camera, an image pick-up apparatus mounted on a cellular phone, or the like is provided with a shutter unit that opens and closes an aperture for exposure arranged between a lens and an image pick-up device. A shutter unit used in a compact type digital camera or a cellular phone opens and closes an aperture by driving an electromagnetic actuator to rotate one or more shutter blades.
For example, Japanese Laid-Open Patent Publication (Kokai) No. 2003-005252 (JP 2003-005252A) discloses a shutter unit that drives a pair of shutter blades by an electromagnetic actuator.
However, as described below, the shutter unit of the above-mentioned publication has a problem that an exposure distribution in a picked-up image is asymmetrical.
FIG. 13 is a view showing a closing travel operation of the shutter blades in the conventional shutter unit that is described in the above-mentioned publication. A pair of shutter blades 30 and 50 are driven by an electromagnetic actuator (not shown), rotate about respective rotating shafts in opposite directions, and open/close a substantial aperture W of the shutter unit that defines a shape of a shooting screen. In a digital camera that allows digital preview, the shutter is open until a release button is pushed. When the release button is pushed, pixels start to accumulate electric charge unison. The shutter closes after the elapse of time defined by a setting shutter speed, and the accumulated electric charge is outputted as a signal.
The rectangular aperture W is equally divided into four, and a first quadrant A, a second quadrant B, a third quadrant C, and a fourth quadrant D are set up. As shown in FIG. 13, when the shutter blades 30 and 50 close the aperture W, the first quadrant A is closed first, the third and fourth quadrants C and D are closed next, and then the second quadrant B is closed.
Accordingly, as shown in FIG. 14, the exposure of the second quadrant B becomes extremely larger than the exposure of the first quadrant A, the third quadrant C, and the fourth quadrant D. This increases necessity of an electrical correction. However, a distribution of the exposure is asymmetrical not only about a center line in a direction of a short side of the aperture W but also about a center line in a direction of a long side. Therefore, since volume of correction data becomes large and a process becomes complicated, the process needs much time.